This invention relates to signal demodulators and, more particularly to digital signal demodulator systems which include an analog-to-digital sampler to provide sampled data in quadrature.
The rapid development of the digital computer art has led to the widespread adoption of digital techniques in the electrical arts. It has, for example, been proposed to employ digital computers in communications systems. However, this application of the digital computer has been limited to the use of the computers as monitoring or control devices. The high speed and flexibility of the digital computer has not, in the past, been fully exploited by incorporating the computer as an integral part of the communication system itself.
The inroduction of digital technology in the communications arts has required the development of analog-to-digital samplers to sample analog signals and provide the samples in digital form. The sampling rate used in such systems is determined by the modulation frequency or bandwidth, not by the carrier frequency used to convey the modulation. Thus, as long as the sampling aperture is narrow as compared to the carrier period, the sampling rate may be approximately the same as the bandwidth of the modulation. With presently available circuit components bandwidths in excess of 10 MH.sub.z may be processed by such digital means using the principles of the present invention.
There has also been a growing interest in a mathematical approach to communications systems. This has led to the development, for example, of phase-locked-loop demodulation systems of the type shown in U.S. Pat. No. 3,099,798 which are widely used in communications systems to provide amplitude, phase, and frequency demodulation. Such systems perform a linear detection of phase and amplitude over a limited range of phase shift and are therefore limited in application to narrow bandwidth signals not subject to wide frequency deviations. Systems of this character are also easily unlocked and require the use of analog circuitry to perform the detection.
Another mathematical approach is suggested in U.S. Pat. No. 3,045,180. This patent teaches that a signal could be analyzed by means of the use of two quadrature channels to provide sine and cosine functions of the signal which are applied to a rectangular-to-polar coordinate converter providing outputs related to the amplitude and phase of the signal. The system shown in this patent, however, employs analog techniques and an electro-mechanical servo loop. It is therefore limited to low frequency applications.
A generally similar system is disclosed on page 129 of Viterbi, Principals of Coherent Communication, published in 1966 by McGraw-Hill Book Company, New York as an estimator of the phase of a sinusoid of known amplitude and frequency with uniform prior-phase probability density. Viterbi also discloses on page 275 the use of a circuit of this character as an element on a frequency estimator.